This invention relates to controlling the temperature coefficient of resistivity (TCR) in resistors. More particularly, it relates to the utilization of vanadium oxide in cermet type resistors to control TCR wherein a distinct advantage is realized in employing a particular glass frit in conjunction with ruthenium and iridium dioxides.
The mechanisms which control or alter the thermostability of cermet resistors is not completely understood. It has been observed that various semiconducting oxides exert an influence on the temperature response of resistivity of cermet resistors so as to make them more thermally stable. Prior to this invention, only resistors described in the electronics industry as thin film resistors have displayed low TCRs. In U.S. Pat. Nos. 2,950,995; 2,950,996 and 3,516,949 vanadium oxide is used in conjunction with noble metal metallizing compositions in relatively small amounts to prevent agglomeration of the metal particles and to improve the solderability, conductivity and/or adhesion properties of the metallizing materials. The same indication of improvement in solderability for these compositions by adding vanadium pentoxide is also indicated in U.S. Pat. No. 3,440,182.
In U.S. Pat. No. 3,553,109 vanadium pentoxide is utilized to control TCR in a resistor composition of the bismuth ruthenate type which utilizes a glass frit binder consisting of 80% lead oxide, 10% silicon oxide and 10% boron oxide. A glass was prepared from the teachings of this particular patent and combined with a conductive phase used to fabricate the resistors of this invention composed of ruthenium dioxide, vanadium pentoxide, and aluminum trioxide as set forth in Example 11. It had a sheet resistivity of 5.49K ohm/sq./mil. and a TCR of +170 .+-. 10 ppm/.degree. C when measured between +25 and -55.degree. C and a +270 .+-. 10 ppm/.degree. C when measured between +25 and +150.degree. C. These results clearly indicate that a low TCR cannot be obtained with ruthenium dioxide and vanadium pentoxide which are the preferred materials of this invention when utilized with the glass described in this particular patent. An attempt was also made to prepare a low TCR resistor material utilizing a purchased glass containing 11% calcium oxide, 44.1% lead oxide, 4.0% aluminum trioxide, 5.5% boron trioxide and 35.4% silicon dioxide. This glass material was combined with a conductive material composed of ruthenium dioxide in an amount of 5.34 weight percent prepared from ruthenium resinate containing 5.26 weight percent ruthenium dioxide, iridium dioxide in an amount of 7.2 weight percent prepared from iridium resinate containing 6.99 weight percent iridium dioxide, 2.95 weight percent bismuth trioxide, 4.18 weight percent vanadium pentoxide and the previously described glass in the amount of 80.41 weight percent. The resistive material prepared had a sheet resistivity of 24,000 ohms/sq./mil. and a TCR of -160 .+-. 10 ppm/.degree. C when measured between +25.degree. C and -55.degree. C and a -50 .+-. 10 ppm/.degree. C when measured between +25.degree. C and +150.degree. C which is considered poorer than when using the materials of this invention.
It is an object of the present invention to provide a novel resistor composition wherein the temperature coefficient of resistivity is held within a narrow plus and minus range over a broad temperature range. It is another object of this invention to provide a low temperature coefficient of resistivity for a cermet material wherein a vanadium oxide is combined with ruthenium and iridium dioxides in designated quantities. It is still another object of this invention to provide a cermet type resistor with a low TCR which is accomplished by employing vanadium oxides with a particular glass frit. It is yet another object of this invention to provide a low TCR cermet resistor which can be produced by current methods of manufacture and can employ either oxide or metallic resinate precursor materials for both the noble metal oxides and the vanadium oxide.